1. Field of the Invention
The present invention relates to a honeycomb structure and a composite honeycomb structure that are widely used as catalyst carrier, manufacturing method thereof, as well as to an air cleaner and a water purifier using the same. The present invention also relates to a translucent honeycomb structure and a technique of removing and decomposing harmful substance using the translucent honeycomb structure.
2. Description of the Background Art
A porous material has numerous small pores and, therefore, the porous material has very large inner surface area relative to the outer surface. Therefore, the porous material has been widely used as an adsorbent, a catalyst or catalyst support, a chromatography column, or a filter for an air conditioner or a water purifier. Such a porous material may be used in various shapes fit for the intended applications, including powder, particles, fiber, honeycomb, thin film and nano tube.
When the porous material is used as a filter for an air cleaner, the most popular porous material is activated carbon. When the activated carbon in the shape of particles is used, the activated carbon particles are filled in a container and the fluid is passed therethrough, so that the fluid is processed. The method of fluid processing, however, is disadvantageous as the pressure loss is large and the linear velocity cannot be increased, though the contact area is very large.
In order to reduce the pressure loss, a porous material having a honeycomb structure with a straight flow path is used. Most of the porous materials having the honeycomb structure currently in use are fabricated by extrusion of ceramics. Generally, when cell density (number of cells per 1 square inch) becomes higher and the thickness of honeycomb wall (wall separating pores) becomes thinner, the contact area with the fluid to be processed increases and, as a result, performance is improved. Further, according to a manufacturing method disclosed in Japanese Patent Laying-Open No. 2004-307294, it is possible to fabricate a honeycomb structure of silica wet gel having pore diameter (channel) controlled in the range of 5 to 50 μm and having large specific surface area of 800 to 900 m2/g.
The honeycomb structure obtained by this method allows separating operation and adsorption because of its fine pore diameter. The honeycomb structure, however, is not surface-modified by a catalyst or the like in the form of metal fine particles, for detoxifying a harmful substance. Further, as the pore diameter of the honeycomb structure is as small as 5 to 50 μm, the problem of pressures loss described above cannot fully be solved.
Further, conventionally, a technique of decomposing a harmful substance utilizing photocatalytic effect generated when a photocatalyst represented by titanium oxide is irradiated with light has been known. The technique is applied to a filter of an air cleaner and the like. Titanium oxide particularly has high catalytic function as regards decomposition of harmful substance. Crystalline titanium oxide, however, has small specific surface area, and its surface activity is low. Therefore, in most cases, titanium oxide in the form of fine powder is used. Titanium oxide fine powder, however, is hard to handle, and therefore, it is used carried by a carrier such as silica gel. Use of a carrier in the form of a honeycomb structure or a sheet has been preferred. As described above, most of the honeycomb structures widely used at present are fabricated by extrusion of ceramics.
The method of irradiating the photocatalyst with light differs dependent on whether the photocatalyst carrier has the honeycomb structure or the sheet shape. By way of example, when the photocatalyst is carried by a honeycomb structure, it follows that the light is directed parallel to the pore diameter of the honeycomb structure, resulting in a fatal disadvantage that only a limited part of the photocatalyst can be activated. When the photocatalyst is carried by a sheet type carrier, the amount of photocatalyst to be activated cannot be increased unless the area of irradiation is widened, which leads to a disadvantage that the apparatus becomes large. Further, when the sheet type carrier is used, the amount of harmful substance that can be processed at one time is limited by the photocatalyst at the outermost surface carried by the sheet, and therefore, the sheet type carrier also has a problem in processing capability.
In order to solve the above-described problems, Japanese Patent Laying-Open No. 2001-269586 proposes fabrication of a translucent honeycomb structure by extrusion, using fine particles of 0.5 to 40 μm of quartz glass, alumina, YAG and yttria as raw materials.
What is provided by this technique is a honeycomb structure formed by extrusion, in which the pore diameter size and honeycomb wall thickness are both controlled in the order of mm. Specifically, the honeycomb structure comes to have smaller surface area per unit volume or unit weight, and hence, it becomes difficult to cause efficient reaction between the fluid containing the harmful substance and the photocatalyst fine particles. The photocatalytic effect of photocatalyst represented by titanium oxide comes from the reaction at the outermost surface of photocatalytic particles, and therefore, it is apparent that the larger the amount of catalyst carried per unit weight or volume of the carrier, the higher the effect.
Further, there may be applications other than the example of air cleaner described above. By way of example, when a translucent honeycomb structure carrying photocatalyst is used for purifying water such as daily life water, it becomes possible to decompose and remove harmful substance contained in the fluid by filtering with irradiation of external light beam, such as ultraviolet ray.